Conventional methods for removing PM contained in exhaust gases emitted from an internal combustion engine, such as a diesel engine, include the method of placing a honeycomb filter made of a heat-resistant ceramic, such as silicon carbide, aluminum titanate or cordierite, in an exhaust system, collecting PM on the honeycomb filter to remove PM from the exhaust gases, and then, upon deposition of a predetermined amount of PM thereon, applying heat to the honeycomb filter to decompose PM by combustion.
However, the combustion temperature of PM is as high as 550 to 650° C., which presents the problem that a device for purifying exhaust gases has a large overall size and the problem that the energy cost for heat application becomes high. Therefore, a honeycomb filter is generally used in which a catalyst for combusting PM is supported on its surface.
With this method, the combustion temperature of PM can be reduced by catalysis, so that the energy taken to apply heat to the honeycomb filter can be reduced. Precious metals, such as platinum, are known as such catalysts, but the amount of production thereof is extremely small, which carries a risk of significant variations in supply-demand balance and price.
To cope with this, Patent Literature 1 proposes an exhaust gas purifying catalyst made of at least one of silicates, aluminates, and zirconates of alkali metals. Patent Literature 2 proposes an exhaust gas purifying catalyst that contains a metal oxide and a sulfate of an alkali metal and/or a sulfate of an alkaline earth metal. Patent Literature 3 proposes an exhaust gas purifying catalyst that is composite oxide particles containing an alkali metal, Si, and Zr.